Physiology of POTS

POTS (Postural Orthostatic Tachyardia Syndrome) is a form of Dysautonomia, or a dysregulation of the autonomic nervous system.  This is a simplified physiology lesson about how POTS effects the body.  

How does POTS affect the body? 

The autonomic nervous system (which controls your body's automated processes) has two systems:

1. Parasympathetic Nervous System: This is your "Rest and Digest" system.  It is helps your body relax and controls your digestion process.  
2. Sympathetic Nervous System: This is your "Fight or Flight" system.  

The sympathetic nervous system is activated when you are scared or surprised.  For example, if someone sneaks up on you and scares or surprises you, how does your body react?  It gets prepped to fight or run!  To do this your body releases adrenaline which signals the heart to beat faster.  Your heart starts beating faster and more blood is diverted to your muscles in preparation for your fight or run.  You may also notice that you get hot and flushed, and it's often hard to think because your body is prepped to react first, think later.  

Most POTS patients have an overactive sympathetic nervous system and/or an underactive parasympathetic nervous system.  That frantic feeling that comes when someone surprises you, is a constant feeling for many of us with POTS.  This is one reason that POTS is so commonly misdiagnosed as anxiety.  Many of the symptoms are similar--fast heart rate, flushing, frantic feelings.  (Studies have shown that anxiety is no more common in Dysautonomia patients than it is in the general population.)  It is the faulty trigger of the sympathetic nervous system that causes these similar manifestations. 

Source: https://commons.wikimedia.org/wiki/File:The_Fight_or_Flight_Response.png

The main characteristic of POTS is a rapid heart-beat (tachycardia).  But why does it beat so fast? 

Normally, when someone stands up or increases movement, the brain sends a signal to the adrenal glands to release adrenaline, which then triggers your heart to beat faster.  At the same time, the brain signals to your veins to constrict, in order to return blood to the heart where it can receive oxygen and re-circulate throughout the body.   The point of this physiological process is to increase your blood flow in order to meet the increased demands for oxygen to your muscles, while maintaining a steady supply to your brain, heart, and other organs. 

In POTS patients, the process just described is largely ineffectual.  For one thing, venous return is often inhibited.  Thus, blood pools in the legs and feet and does not return to the heart efficiently in order to be re-oxygenated and recirculated throughout the body.  Thus, the stroke volume (output of blood from the heart) is reduced.  When the brain fails to receive enough oxygenated blood, it mistakenly thinks "oh, my heart needs to beat faster to meet my demands," thus, it signals for more adrenaline to be released, which in turn triggers the heart to beat even faster, but the heart pumping faster doesn't do much good when there is not enough blood to pump.  As the brain receives less and less oxygen, it continues to signal adrenaline to trigger a faster heart rate, which becomes increasingly ineffective and only produces negative side effects.  As you can see, it's another vicious cycle. 

Without enough oxygenated blood going to the brain and other organs, patients may experience brain fog, light headedness, dizziness, pre-syncope, syncope (fainting), shortness of breath, chest pain, poor motor coordination, fatigue, migraines, sensory overload, and more.  The excess adrenaline coursing through the body can cause tremors or myoclonic jerks, flushing, nerve pain, heart palpitations, intense fatigue, agitation, headaches, and insomnia.  It also prevents the parasympathetic nervous system from activating-- which also contributes to sleeplessness and digestion problems. 

Unfortunately, the body is not designed to be in this sympathetic state for long periods of time- and it is exhausting!  I've heard it compared to slamming on the gas and the brakes at the same time.  (If you've ever had an epi-pen or adrenaline shot administered-- it is a similar experience).

It can also be noted that studies have shown that a large number of POTS patients have similar contributing factors such as lower blood volume, smaller heart sizes, hypermobility, and increased sensitivities to food and environmental allergens (mast cell disorder)-- all of which would exacerbate the problems caused by the autonomic system's dysregulation.  For example, lax connective tissue from hypermobility or EDS (Ehlers Danlos Syndrome) can contribute to the poor constriction of veins, reducing the return of blood to the heart.  

Want to know more?

Read What is POTS?

Learn more about Living with POTS